Biceps: when the muscle shortens, points (a) insertion and (b) origin are brought closer together and the arm is bent, or flexed at the elbow.

Antique apparatus for recording successive muscular contractions


Antique apparatus for recording successive muscular contractions

Sliding Filaments

Myofibrils comprised of actin and myosin myofilaments

Myofibrils actin myosin myofilaments with I and H bands, H zone, Z line

Tension Potential

A muscle's ability to contract is dependent upon its length, or degree of contraction. A muscle can contract more forcefully when it is slightly stretched. Muscle generates maximal concentric tension at a length 1.2 times its resting length. Beyond this length, active tension decreases due to insufficient sarcomere overlap (Norkin & Levangie 1992). A muscle tension becomes weaker as it nears complete contraction. See muscle length-tension relation graph below. Also see active insufficiency and passive insufficiency below.

Active Insufficiency


The inability for a biarticulate muscle to exert enough tension to shorten sufficiently to complete full range of motion in both joints simultaneously. Active insufficiency explains the quantity of cross bridges active from the myosin to the actin dependent upon the muscle's length. Also see tension potential above and muscle length-tension relation graph below. These diagrams illustrate how hamstring involvement is influenced by the position of the knee. In the first figure, the hamstrings are in a mechanical advantage during hip extension when the knee is straight or nearly straight. In the second figure, the hamstring muscles are relaxed when the knee is bent, particularly as the hip is further extended; the gluteus maximus are thereby more exclusively involved in hip extension. See examples of how body seemingly inadvertently compensates for active insufficiency during certain weight training exercises.

Passive Insufficiency

The inability for a biarticulate muscle to stretch enough to complete full range of motion in both joints simultaneously. Also see tension potential above and muscle length-tension relation graph below. The above diagrams also illustrate how flexion at the hip joint is influenced by the position of the knee. In the first figure, the hamstrings are tightly stretched when the knee is straight. In the second figure, the muscles are relaxed when the knee is bent; a greater amount of hip flexion is thereby permitted. When one stretches a biarticulate muscle, they are essentially placing it into a position of passive insufficiency. Although passive insufficiency explains range of motion limitations, it also implies the weaker elongated position in which the muscles placed. See examples of how body seemingly inadvertently compensates for active insufficiency during certain weight training exercises.

Muscle Length-tension


Muscle Length-tension Relationship

Greatest tension is developed at point B (slightly stretched) with less tension developed at points A (contracted) and C (stretched).

Muscle Fatigue & Blood Supply



Musccle Fatique and Blood Supply

Muscle Fatigue (a) muscle with intact circulation (b) isolate muscle. Note sustained muscular contraction may occlude local vasculature momentarily impeding blood flow to activated muscle. Also see Pump and Burn.

Stretch-shortening Cycle

Muscular forces can be enhanced when the muscle is stretched (causing eccentric tension) immediately before concentric contraction. This stretch-shorten cycle (SSC) occurs naturally in running, jumping, and all other activities in which muscles are suddenly stretched by impact or other external forces. The stretch-shortening cycle saves energy by temporarily storing potential energy through elastic recoil resulting from an external quick stretching force.

  • Plyometric vertical jumps following quick landings from a 0.4 m height have been found to be higher than made from a semisquat, with or without a dip or countermovement (Asmussen & Bonde-Peterson 1976). Also see Depth Jump Heights and Speed of Contraction.
  • Recoils or windups immediately before throwing, striking, or kicking can augment the muscle contraction forces to come.
  • In running, the shock absorption occurring at the foot plant contributes to greater force production at push-off (Cavagna, Dusman & Margaria 1968).
  • In sprint running, the quick stretch (0.15 to 0.1 seconds) applied at ground contact during push-off increased force production (Kreighbaum 1996).
  • In weight training, even a one second delay at the bottom of a bench press would result in a 55% loss of force.

Slow Oxidative (SO) fibers have greater elastic properties than Fast Glycolytic (FG) (Kimi & Bosco 1978, Milner 1988)

Fibers Types

  Slow oxidative (SO) Fast Oxidative Glycolytic (FOG) Fast Glycolytic (FG)
  Type I (red) Type IIA (white) Type IIX (white)
Speed of contraction Slow Fast Fast
Force of contraction Low Medium High
Anaerobic capacity Low Medium High
Aerobic capacity High Medium Low
Capillary density High Medium Low
Mitochondrial density High High Low
Motor neuron size Small Medium Large
Major substrate Triglycerides CP, Glycogen CP, Glycogen
Activity Prolonged low intense Prolonged high intense Short high intense
Average fiber percentage 50% 35% 15%
  • Individual
    • Ratio of both types of muscle fiber varies in each individual, thought to be genetically determined.
  • Sex
    • Simoneau, et al. (1985) found differences in ratios of fiber types between sedentary men and women. Other studies suggest fiber type variations may be largely due to differences in physical activity (Miller AEJ et al. 1992).
    • In order from greatest to smallest area occupied in muscle
      • Men: type IIA, I, IIX
      • Women: type I, IIA, IIX
    • Women's muscle fiber area of both type I and II are smaller than men's
  • Fiber Ratio
    • Ratio of both types of muscle fiber varies in each muscle.
      • Eg: Percentage of Type I Fibers: Quadriceps (~52%), Soleus (~80%), Orbicularis Oculi (~15%)
    • Muscles that primarily maintain posture against gravity require more endurance and generally have a higher percentage of slow-twitch fibers.
    • Muscles that produce powerful, rapid, explosive strength movements tend to have a greater percentage of fast-twitch fibers.
  • Recruitment Velocity
    • Recruitment velocity is the rate at which a muscle fiber can achieve maximum tension
    • Recruitment velocity varies from 20 milliseconds for white fibers to 65 milliseconds for red fibers.
  • Sport
    • Sprinters and weight lifters have a large percentage of fast-twitch fibers.
    • Marathon runners generally have a higher percentage of slow twitch fibers.
  • Training
    • Both types of fibers can improve their metabolic capabilities through specific strength and endurance training.
  • Nomenclature
    • Early researches believed humans possessed type IIb.
    • Later research revealed IIB was in fact IIX in humans.
    • Nonhuman fiber types include true IIb fibers as well as others (IIc, IId, etc.)


Muscle Tendon

One end of muscle fiber showing attachment of tendon to sarcolemma

Muscle Nuclei

Part of a muscle fiber specially prepared to bring out the numerous nuclei

Muscle Artery

An artery branching into capillaries between three muscle fibers

Muscle EMG Strip

Record of successive contractions of elbow flexor muscles

Muscle as an Endocrine Organ

Working muscle releases myokines, a type of cytokines that regulate the metabolism of muscles and other tissues and organs including the adipose tissue, liver, and brain via their respective receptors. The functions of the various types of myokines vary:

IL-6: < Inflammation, > Muscle atrophy, > Fatty acid Oxidation
BDNF: > Muscle regeneration, > Fatty acid oxidation
IL-15: > Fat metabolism, > Myoblast differentiation, > Muscle mass / atrophy
SPARC: > Muscle repair
FGF21: > Muscle Mass, Mitochondrial biogenesis
Deorin: > Myhogeneis, < Muscle atrophy
Myonectin: < Autophagy, > Mitochondrial biogenesis
Myostatin: > Muscle atrophy, < Muscle mass
Irisin: > Muscle mass, > Muscle hypertrophy, > Fatty acid oxidation

Lee JH, Jun HS (2019).  Role of Myokines in Regulating Skeletal Muscle Mass and Function. Front. Physiol.

Musculotendinous Receptors

Golgi Tendon Organs

Golgi Tendon Organs (GTO) are receptors located in tendon merging with its muscle (myotendinous junction) and positioned in line with the direction of muscle contraction. These receptors are sensitive to both the passive and active pull of muscle tendon (Gregor, 1989). The GTO inhibits the contraction of the associated muscle (autogenic inhibition) and excites the antagonistic muscle group. Also see Golgi Tendon Organs Question/Answer.

The difference between detection and sensing is through Golgi tendon organs (GTOs). Joint receptors detect kinesthesia (sense of movement) whereas GTOs sense the tension generated by muscle (Macefield & Knellwolf 2018). Golgi tendon organs are spindle-shaped end organs that are found at the transition of muscle fibers and tendons (Goodman & Bensmaia 2020). Golgi tendon organs are arranged in series, unlike muscle spindles which are arranged in parallels. There are two afferent groups of the GTOs:


  1. Group 1a afferents
  2. Group 1b afferents

The GTO afferent, group 1b afferent, innervates each individual GTO which is distributed through many terminal branches. The 1b afferents respond to the muscles tension, signaling the production of muscle force and are insensitive to passive stretch (Goodman & Bensmaia 2020).

Muscle Spindles

Muscle spindles are a type of receptor located throughout the muscle and situated between and parallel to individual muscle fibers. In this position, they are stretched along with adjacent muscle fibers, causing a reflex contraction of their associated host muscle, also known as myotatic or stretch reflex. The knee-jerk reflex is an example. Both the stretch reflex and recoil of stored elastic energy used in striking or throwing movements. Muscle spindles also are involved in motor control, providing constant monitoring and regulation of sensorimotor function, enabling appropriate body movement, both reflective and voluntary (Schmidt 1988, Gregor 1989).

Muscle spindles are dense, stretch-sensitive mechanoreceptors within skeletal muscle that contribute to proprioception (Macefield & Knellwolf 2018). Muscle spindles are located in parallel with extrafusal skeletal muscle fibers. There are three categories of muscle spindles, two proprioceptive afferent fibers and three afferent classes:


  1. Bag 1 fibers
  2. Bag 2 fibers
  3. Nuclear chain fibers


  1. Group 1a
  2. Group 2


  1. Type 1a
  2. Type 1b
  3. Type 2

In relation to muscle, group 1a afferents are sensitive to velocity of muscular stretching (Goodman & Bensmaia 2020). Group 2 afferents are sensitive to the degree of muscular stretch. Due to the arrangement of both muscle spindles and GTOs, their position with respect to the muscle explains their purpose (Goodman & Bensmaia 2020).

Muscle spindles are arranged in parallels to the muscle, explaining their sensitivity to muscle stretch. Golgi tendon organs are arranged in series to the muscle, explaining their sensitivity to muscular tension. (Goodman & Bensmaia 2020)

Interestingly, the placement of GTOs further explains afferents sensitivity to active force generation by the muscles. When the muscle is not actively contracting, muscle stiffness is less than tendon stiffness, thus the muscle experiences a majority of the strain. When a muscle is actively contracting, the stiffness of the muscle approaches the stiffness of tendon, thus displaying greater tendon strain. (Goodman & Bensmaia 2020)

Goodman J, Bensmaia S (2020). The Neural Mechanisms of Touch and Proprioception at the Somatosensory Periphery. The Senses: A Comprehensive Reference, 2-27.

Macefield V, Knellwolf T (2018). Functional properties of human muscle spindles. Journal Of Neurophysiology, 120(2), 452-467.

Also see Motor Unit Recruitment and Golgi Tendon Organs Discussion.

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